Thaw sirloin cap roast in refrigerator overnight.
Remove from refrigerator and pat dry with paper towels, then use a sharp knife cut a crosshatch pattern into the fat layer without cutting into the meat and rub down with grapeseed oil.
Generously season roast on all sides with sirloin roast rub and let come to room temperature over about 40 to 45 minutes.
Pre-heat grill to 250 degrees Fahrenheit indirect heat.
Place thermometer probe in the center-most thickest part of roast and cook until 15 degrees below desired doneness for medium-rare (130 degrees Fahrenheit); this should take about 40 to 45 minutes. For different steak doneness, refer to the Omaha Steaks cooking chart.
Remove roast from grill and increase grill temperature to 550 degrees Fahrenheit direct heat.
Place roast back down on grill fat-cap-side-down for about 1 minute until browned sear forms; flip roast meat-side-down and sear for about another minute until browned; sear each edge about 30 seconds per side until browned; roast should be 5 degrees below desired doneness at this point.
Remove roast from grill and rest on cutting board for 15 minutes before slicing against the grain.
Serve with harissa crema.